A TFT-LCD has the advantages of portability, low power consumption, and low radiation, and has been widely used in various portable information products such as notebooks, personal digital assistants (PDAs), video cameras and the like. Furthermore, the TFT-LCD is considered by many to have the potential to completely replace CRT (cathode ray tube) monitors and televisions.
The TFT-LCD usually includes a color filter (CF) substrate, a thin film transistor (TFT) array substrate, and a liquid crystal layer sandwiched between the two substrates. The TFT array substrate includes a plurality of gate lines that are parallel to each other and extend along a first direction, and a plurality of data line that are parallel to each other and extend along a second direction orthogonal to the first direction. The smallest rectangular area formed by any two adjacent gate lines together with any two adjacent data lines defines a pixel region thereat.
In each pixel region, a TFT is provided in the vicinity of a respective point of intersection of one of the gate lines and one of the data lines. The TFT functions as a switching element. A pixel electrode is connected to the TFT. The CF substrate includes a plurality of common electrodes, each common electrode corresponding to a respective one of the pixel electrodes on the TFT array substrate.
When the TFT-LCD works, gradation voltages are applied to the pixel electrodes and a common voltage is applied to the common electrodes. Thus an electric field is applied to the liquid crystal molecules of the liquid crystal layer. At least some of the liquid crystal molecules change their orientations, whereby the liquid crystal layer provides anisotropic transmittance of light therethrough. Thus the amount of the light penetrating the CF substrate is adjusted by controlling the strength of the electric field. In this way, desired pixel colors are obtained at the CF substrate, and the arrayed combination of the pixel colors provides an image viewed on a display screen of the TFT-LCD.
If an electric field between the pixel electrodes and the common electrodes continues to be applied to the liquid crystal material in one direction, the liquid crystal material may deteriorate. Therefore, in order to avoid this problem, gradation voltages that are provided to the pixel electrode are switched from a positive value to a negative value with respect to the common voltage. This technique is referred to as an inversion drive method.
However, the common voltage may vary in different environmental temperatures. But the inversion drive method needs the common voltage to be a predetermined constant value in order to prevent appearing flicker on the screen of the TFT-LCD. Thus a common voltage adjusting circuit is needed.
FIG. 2 is a diagram of a typical common voltage adjusting circuit of a TFT-LCD. The common voltage adjusting circuit 100 includes a power supply V1, an output terminal V0, a first resistor R1, a second resistor R2, a third resistor R3, a fourth resistor R4, a fifth resistor R5, a sixth resistor R6, a seventh resistor R7, an eighth resistor R8, a ninth resistor R9, a tenth resistor R10, a first switch S1, a second switch S2, a third switch S3, a fourth switch S4, and a comparator 10. Each of the switches S1, S2, S3, S4 includes a first terminal 1, a second terminal 2, and a third terminal 3.
The ninth resistor R9 is connected between the power supply V1 and ground. The fourth resistor R4, the first resistor R1, the second resistor R2, and the third resistor R3 are connected in series between ground and the power supply V1, wherein the fourth resistor R4 is connected directly to ground.
A connecting node between the first resistor R1 and the fourth resistor R4 is connected to the third terminal 3 of the first switch S1 via the fifth resistor R5. A connecting node between the first resistor R1 and the second resistor R2 is connected to the third terminal 3 of the second switch S2 via the sixth resistor R6. A connecting node between the second resistor R2 and the third resistor R3 is connected to the third terminal 3 of the third switch S3 via the seventh resistor R7. The power supply V1 is connected to the third terminal 3 of the fourth switch S4 via the eighth resistor R8. The second terminals 2 of the switches S1, S2, S3, S4 are connected to ground. The first terminals 1 of the switches S1, S2, S3, S4 are connected to a noninverting input of the comparator 10. An inverting input of the comparator 10 is connected to ground. The output of the comparator 10 is connected to the output terminal V0. The third terminals 3 of the switches S1, S2, S3, S4 are used to receive four binary signals B0, B1, B2, B3 respectively.
Resistances of the first resistor R1, the second resistor R2, and the third resistor R3 are equivalent to each other. Resistances of the fourth resistor R4, the fifth resistor R5, the sixth resistor R6, the seventh resistor R7, the eighth resistor R8, and the ninth resistor R9 are equivalent to each other.
When four binary signals B0, B1, B2, B3 are equal to “1” respectively, the third terminal 3 and the first terminal 1 of each switch S1, S2, S3, S4 is electrically connected. The output of the comparator 10 provides the maximal adjusting voltage to the output terminal V0. The potential of the maximal adjusting voltage is approximately equal to that of the power supply V1.
When the four binary signals B0, B1, B2, B3 are equal to “0” respectively, the third terminal 3 and the second terminal 2 of each switch S1, S2, S3, S4 are electrically connected. The output of the comparator 10 provides a minimal adjusting voltage to the output terminal V0. The potential of the minimal adjusting voltage is approximately equal to zero volts.
When the four binary signals B0, B1, B2, B3 are different values respectively such as “0” or “1”, the output of the comparator 10 provides a middle adjusting voltage to the output terminal V0. The potential of the middle adjusting voltage is in the range of 0-V1. Thus the common voltage adjusting circuit 100 transforms different binary signals B0, B1, B2, B3 therein, for respectively adjusting voltages and providing the adjusting voltages to control the common voltage of the TFT-LCD.
However, the parameters of the elements of the common voltage adjusting circuit 100, such as the first resistor R1, the second resistor R2, the third resistor R3, and the fourth resistor R4, vary in different environmental temperatures. Therefore voltages respectively at the connecting node between the first resistor R1 and the fourth resistor R4, the connecting node between the first resistor R1 and the second resistor R2, and the connecting node between the second resistor R2 and the third resistor R3 vary with different environmental temperatures. Thus, a voltage provided to the noninverting input of the comparator 10 cannot be accurately controlled, and the adjusting voltage generated by the comparator 10 cannot be accurately controlled.
What is needed, therefore, is a common voltage adjusting circuit of a TFT-LCD that can overcome the above-described deficiencies.